1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor gas rate sensor for detecting an angular velocity acting on a base.
2. Description of the Related Art
A basic structure of a semiconductor gas rate sensor will be described below with reference to FIGS. 1 through 3 of the accompanying drawings.
The semiconductor gas rate sensor shown in FIG. 1 comprises a chip-like base 6 composed of a first semiconductor substrate 1 and a second semiconductor substrate 3 with a thermosetting adhesive layer 2 disposed on a mating surface thereof. The first and second semiconductor substrates 1, 3 are bonded to each other by the thermosetting adhesive layer 2 interposed therebetween. As shown in FIG. 2, the base 6 has a gas flow passage 4 defined therein and a nozzle port 5 defined therein for ejecting a gas into the gas flow passage 4. A pair of heat wires 7, 8composed of thermosensitive resistor elements, respectively, is disposed in and extends across the gas flow passage 4. The heat wires 7, 8 serve as a detector for detecting a deflected state of a gas flow at the time an angular velocity acts on the base 6. The base 6 also has a gas inlet hole 9 defined therein for connection to a micropump (not shown), and a gas reservoir 10 defined therein between the gas flow passage 4 and the gas inlet hole 9 for reducing pump-induced pressure pulsations of the gas introduced from the gab inlet hole 9 to allow a stable gas flow to be supplied from the nozzle 5 into the gas flow passage 4.
The semiconductor gas rate sensor is a very small, precision device having a thickness of about 1200 .mu.m, with the nozzle 5 having a width of about 800 .mu.m. Heretofore, the semiconductor gas rate sensor has been manufactured as follows:
First, heat wires 7, 8 of platinum or the like are evaporated on the mating surface of the first semiconductor substrate 1 (see FIG. 3). Then, the mating surface of the first semiconductor substrate 1 is etched to define therein a recess 11 which serves as a portion of the gas flow passage 4, a recess 12 which forms the nozzle 5 when combined with a mating surface of the second semiconductor substrate 3, and a recess 13 which serves as a portion of the gas reservoir 10. Specifically, the recess 11 is formed such that a bridge 18 extends across the recess 11, and the heat wires 7, 8 are placed on the bridge 18 so as to extend across the gas flow passage 4.
Thereafter, the mating surface of the second semiconductor substrate 3 is etched to define therein recesses 15, 16 which form the gas flow passage 4 and the gas reservoir 10, respectively, when combined with the recesses 11, 13 in the first semiconductor substrate 1. The recess 16 is defined in communication with the gas inlet hole 9. The adhesive layer 2 which is made of epoxy resin is deposited on the mating surface of the second semiconductor substrate 3.
Then, as shown in FIGS. 2 and 4 of the accompanying drawings, the second semiconductor substrate 3 is joined by the adhesive layer 2 to the first semiconductor substrate 1 which is positioned underneath the second semiconductor substrate 3. Thereafter, while keeping the first semiconductor substrate 1 underneath the second semiconductor substrate 3, the first and second semiconductor substrates 1, 3 are heated to set the adhesive layer 2, whereupon the first and second semiconductor substrates 1, 3 are firmly bonded together, completing the semiconductor gas rate sensor.
However, since the first and second semiconductor substrates 1, 3 are heated while keeping the first semiconductor substrate 1 underneath the second semiconductor substrate 3, the adhesive layer 2 over the recess 12 is melted and softened in an initial stage of the heating step. The softened adhesive layer 2 then flows toward and sags from an upper corner of the recess 12 due to gravity, and is finally set, as shown in FIG. 5 of the accompanying drawings. The adhesive layer 2 thus set in the sagging state in the recess 12 makes the nozzle 5 asymmetrical in shape, and tends to deflect a gas flow as it is injected from the nozzle 5 into the gas flow passage 4. The gas flow is deflected to a greater extent the larger the amount of gas is. When this happens, the detector composed of the heat wires 7, 8 detects an angular velocity signal even though no angular velocity is acting on the base 6. Consequently, the semiconductor gas rate sensor is defective and cannot be used. The conventional process of manufacturing the semiconductor gas rate sensor suffers from a poor yield.
One solution is to place a spacer (not shown) on the area of the mating surface of the second semiconductor substrate 3 over the recess 12 to eliminate the adhesive layer 2 from only the mating surface area over the recess 12. However, because the recess 12 normally has a width of about 800 .mu.m, it is practically highly difficult to get rid of the adhesive layer 2 from only the mating surface area over the recess 12. Actually, as shown in FIG. 6 of the accompanying drawings, asymmetrical gaps 19 are created between opposite upper edges of the recess 12 and the mating surface of the second semiconductor substrate 3. Inasmuch as the gas leaks into the gaps 19 during operation of the semiconductor gas rate sensor, the gas flow from the nozzle 5 into the gas flow passage 4 is still deflected.